1. Field of the Invention
The present invention relates to a filter circuit, particularly to a filter circuit for converting unbalanced/balanced signals into balanced/unbalanced signals, and here particularly to a filter circuit comprising BAW resonators (BAW=bulk acoustic wafer). Further, the present invention relates to a filter circuit with a plurality of BAW resonators, which enables a transformation of impedance levels between an input port and an output port of the filter circuit.
2. Description of the Related Art
RF filters based on resonators, such as BAW filters, have two basic topologies, which will be discussed in more detail with regard to FIGS. 1 and 2.
The first topology (see FIG. 1) is the so-called ladder filter. The ladder filter 100 comprises an input port 102 with a first input terminal 104 and a second input terminal 106. Further, the filter 100 comprises an output port 108 with a first output terminal 110 and a second output terminal 112. An input signal IN is applied to the first input terminal 104 of the input port 102, and an output signal OUT is applied to the first output terminal 110 of the output port 108. In the filter 100 shown in FIG. 1, two series resonators Rs1 and Rs2 are connected in series between the first input terminal 104 and the first output terminal 110. Further, two parallel resonators Rp1 and Rp2 are provided. The first parallel resonator Rp1 is connected in parallel to the input port 102 as well as in parallel to the first series resonator Rs1. The second parallel resonator Rp2 is connected in parallel to the output port 108 as well as in parallel to the second series resonator Rs2. The second input terminal 106 as well as the second output terminal 112 are connected to a reference potential 114, e.g. ground. The parallel resonators Rp1 and Rp2 are also connected to the reference potential. The conventional filter illustrated in FIG. 1 is a ladder filter with two stages with a single input IN and a single output OUT for transmitting unbalanced signals.
In FIG. 2, a known lattice filter (bridge filter) with one stage (two series resonators and two parallel resonators) is discussed in more detail. In the description of FIG. 2, similar or equal devices already described with reference to FIG. 1 will be provided with the same reference numbers.
The lattice filter 120 receives a balanced input signal IN at the first input terminal 104 and at the second input terminal 106 of the input port 102. A balanced output terminal OUT is output at the output signal 108 at the terminals 110 and 112. A series resonator Rs1 is provided between the first input terminal 104 and the first output terminal 110. Likewise, a series resonator Rs2 is provided between the second input terminal 106 and the second output terminal 112. A first parallel resonator Rp1 is connected between the first input terminal 104 and the second output terminal 112. A second parallel resonator Rp2 is connected between the second input terminal 106 and the first output terminal 110. The filter 120 shown in FIG. 2 is fully differential, i.e. both input ports 102 and 110 are balanced.
Filters with good selectivity and low insertion loss can be produced by using BAW resonators, which are used to construct individual blocks or stages of impedance element filters. These filters have two basic topologies, which will be discussed in more detail with reference to FIGS. 1 and 2.
With regard to the filters described with reference to FIGS. 1 and 2, it should be noted that the series resonators and parallel resonators are preferably BAW resonators, wherein the series resonators and the parallel resonators are produced with a predetermined resonance frequency, respectively. The resonance frequencies of the parallel resonators are preferably off-tune to the resonance frequencies of the series resonators to obtain the desired filter effect. It should be noted that the series resonators and the parallel resonators used in the ladder filter 100 differ from the series resonators and parallel resonators used in the lattice filter 120, particularly in filter circuits with mainly equal filter characteristics but different topology.
However, in the ladder filter 100, there is merely the possibility to receive an unbalanced input signal and to output a respective unbalanced output signal. Also, the lattice filter 120 allows merely the reception of a balanced input signal and the output of a balanced output signal.
There are, however, applications where it is required to perform a transformation/conversion of an unbalanced input signal into a balanced output signal, or a transformation/conversion of a balanced input signal into an unbalanced output signal. Further, there are applications where alternatively or additionally to the conversion of balanced/unbalanced signals into unbalanced/balanced signals different port impedances exist at the inputs and outputs, which also have to be handled.
A traditional method to perform a respective conversion/transformation is to provide an additional component, which is referred to as balun. The balun can either be a magnetic transformer, an LC circuit or a strip-line structure, wherein the balun is disposed on a printed circuit board before or after the filter circuits shown in FIGS. 1 and 2. The usage of discrete baluns before or after the filters is a possibility but increases the number of required components and the required space on the printed circuit board.
In surface acoustic wave filters (SAW filters), an acoustic balancing function can be implemented without additional components, it does, however, decrease the behavior of the overall filter significantly. Further, this balancing function leads to the filter being very sensitive against electrostatic discharges and further the possibilities of manipulating powers are limited drastically, i.e. the transmittable powers across such a filter structure are very low. One example for such an SAW filter is described in JP 2000-114917A. It is another disadvantage of the coupled SAW filter that the response of these filters is generally worse than the response of impedance element filters, particularly the so-called roll-off or the selectivity near the pass band.
One solution for converting unbalanced signals into balanced signals is, for example, described in EP 1 202 454 A according to which filter structures, similar to the ones in FIGS. 1 and 2 are combined, which means the lattice filter is connected to the output of the ladder filter. However, this solution has significant disadvantages for the practical application of such a filter and is particularly disadvantageous in that it can only be related to floating differential loads, which means no RF leaking current against ground is allowed.
In connection with BAW filters, no solution is known that would suggest in what way an impedance transformation could be performed.